Now I know where the phrase "Crossing the Rubicon" originated and I hang my head in shame for not having known before.
In 49 BC, when Julius Caesar was returning to Rome from his conquest of Gaul, his arch rival Pompey moved against him in the senate, stripping him of his rank and had charges of treason brought against him. This forced Caesar to pause with his legions at the river Rubicon, the nominal line that marked the boundaries of Italy and Gaul, where he had to decide his fate: to cross with his armies or obey the senate.
He crossed, allegedly uttering "The die is cast."
This resulted in the Roman civil war, his showdown with Pompey in Greece and his eventual rise to dictator for life.